2012-02-26 20:29:14 -05:00
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PEP: 415
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Title: Implementing PEP 409 differently
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Version: $Revision$
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Last-Modified: $Date$
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Author: Benjamin Peterson <benjamin@python.org>
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Status: Draft
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 26-Feb-2012
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Post-History: 26-Feb-2012
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Abstract
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========
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PEP 409 allows PEP 3134 exception contexts and causes to be suppressed when the
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exception is printed. This is done using the ``raise exc from None``
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syntax. This PEP proposes to implement context and cause suppression
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differently.
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Rationale
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=========
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PEP 409 changes ``__cause__`` to be ``Ellipsis`` by default. Then if
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``__cause__`` is set to ``None`` by ``raise exc from None``, no context or cause
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will be printed should the exception be uncaught.
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The main problem with this scheme is it complicates the role of
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``__cause__``. ``__cause__`` should indicate the cause of the exception not
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whether ``__context__`` should be printed or not. This use of ``__cause__`` is
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also not easily extended in the future. For example, we may someday want to
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allow the programmer to select which of ``__context__`` and ``__cause__`` will
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2012-02-26 20:39:52 -05:00
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be printed. The PEP 409 implementation is not amenable to this.
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2012-02-26 20:29:14 -05:00
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The use of ``Ellipsis`` is a hack. Before PEP 409, ``Ellipsis`` was used
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exclusively in extended slicing. Extended slicing has nothing to do with
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exceptions, so it's not clear to someone inspecting an exception object why
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``__cause__`` should be set to ``Ellipsis``. Using ``Ellipsis`` by default for
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``__cause__`` makes it asymmetrical with ``__context__``.
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Proposal
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========
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2012-02-27 18:59:27 -05:00
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A new attribute on ``BaseException``, ``__suppress_context__``, will
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2012-03-05 16:43:34 -05:00
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be introduced. Whenever ``__cause__`` is set, ``__suppress_context__``
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will be set to ``True``. In particular, ``raise exc from cause``
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syntax will set ``exc.__suppress_context__`` to ``True``. Exception
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printing code will check for that attribute to determine whether
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context and cause will be printed. ``__cause__`` will return to its
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original purpose and values.
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2012-02-26 20:29:14 -05:00
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There is precedence for ``__suppress_context__`` with the
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``print_line_and_file`` exception attribute.
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2012-02-27 10:51:11 -05:00
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To summarize, ``raise exc from cause`` will be equivalent to::
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exc.__cause__ = cause
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raise exc
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2012-03-05 16:43:34 -05:00
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where ``exc.__cause__ = cause`` implicitly sets
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``exc.__suppress_context__``.
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2012-02-26 20:29:14 -05:00
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Patches
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=======
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There is a patch on `Issue 14133`_.
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References
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==========
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.. _issue 14133:
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http://bugs.python.org/issue6210
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Copyright
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=========
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This document has been placed in the public domain.
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..
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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sentence-end-double-space: t
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fill-column: 70
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coding: utf-8
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End:
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